The invention relates to a reducing gear, in particular for a starter device of an internal combustion engine, for speed-reducing coupling of a drive shaft and a driven shaft, by means of at least two gearwheels which engage with one another. The invention also relates to a starter device of an internal combustion engine.
Reducing gears of the type mentioned at the beginning are known from the prior art. They are frequently used in starter devices of internal combustion engines to increase the torque, for example of an electric motor, as a result of which the overall size of the electric motor is kept small and its weight is kept low. If the internal combustion engine is to be started, the electric motor whose torque is reduced by means of the reducing gear—by reducing the speed and thereby setting the internal combustion engine in motion—is operated for this purpose. Starting from a speed which is dependent on the internal combustion engine used, the internal combustion engine can enter an independent operating mode. This procedure at the starting of the internal combustion engine is necessary since internal combustion engines are usually not capable of self-starting. Frequently, gearwheels with conventional toothing are used for the reducing gear, for example a planetary gear mechanism. This has the disadvantage that during the operation vibrations are excited, which results in poor acoustic properties of the reducing gear. The noises which are produced as a result can in certain cases be comparatively loud and therefore disruptive. In order to prevent the production of these vibrations, a helical toothing has now come to be provided for the reducing gear in some cases. This measure improves the engagement conditions and prevents the production of vibrations and the associated noise level. This means improved acoustic properties. However, the helical toothing also means that axial thrust forces act on the gearwheels which are equipped with the helical toothing. As a result of this, the corresponding bearing arrangement of the gearwheels or of the shafts associated with the gearwheels has to be provided depending on the thrust forces which are produced. The rigidity of the housing must also be configured taking into account the thrust forces. This gives rise to a complex design of the bearings and of the housing, which results in an increase in costs. Also, relatively high friction is present at the gearwheels with helical toothing, with the result that the efficiency of the reducing gear is worsened and therefore the power of the starter is reduced. This means that the starter capability of the starter device or of the electric motor is reduced. For this reason, the power of the electric motor must be increased in order to obtain the necessary starting capability. This in turn causes the wear of the electric motor to increase, in particular the wear of the commutation system, and therefore reduces the service life of the electric motor. Furthermore, there would be the possibility of providing the gearwheels with a double helical toothing. This prevents the occurrence of axial thrust forces acting on a bearing since said forces are taken up by the toothing itself. However, the double helical toothing results in the occurrence of further frictional forces and therefore in a further reduction in the efficiency. The fabrication is also complex and therefore expensive. The advantageous vibration behavior of the helical toothing or of the double helical toothing therefore entails a worsening of the efficiency and a more complex design.